This invention relates to a process for the separation and obtaining at least two gaseous components from a gaseous mixture containing same, by scrubbing with a physical scrubbing liquid, i.e., a liquid which can absorb gases without any chemical reaction, e.g., methanol.
Such processes are known from German Pat. No. 1,814,064, as well as DAS [German Published Application] No. 2,548,700, English language equivalents being U.S. Pat. Nos. 3,718,006 and 4,050,909, respectively.
German Pat. No. 1,814,064 relates specifically to the removal of carbon dioxide and hydrogen sulfide from a hydrogen-rich raw gas. This raw gas is treated in a two-stage scrubbing step: the first stage comprises scrubbing the raw gas countercurrently with carbon dioxide-loaded-methanol to scrub out hydrogen sulfide; and the second stage comprises scrubbing the resultant treated raw gas with completely regenerated methanol to scrub out carbon dioxide. A portion of the methanol loaded with carbon dioxide in the second scrubbing stage is withdrawn therefrom and introduced to the first scrubbing stage and, after absorption of the hydrogen sulfide, withdrawn as a first scrubbing liquid stream. The remaining portion of the methanol discharged from the second scrubbing stage and loaded with carbon dioxide but essentially no hydrogen sulfide is discharged from the scrubbing unit as a second stream of scrubbing liquid.
The first scrubbing liquid stream is expanded, with liberation of a gaseous fraction containing carbon dioxide and hydrogen sulfide. This gaseous fraction is then freed of hydrogen sulfide by countercurrent scrubbing with the likewise expanded, second scrubbing liquid stream. (This hydrogen sulfide represents the component more soluble in methanol as compared with the carbon dioxide.) Thus, this treatment results in a first product gas stream containing primarily carbon dioxide which, however, is greatly contaminated with nitrogen since the first scrubbing liquid stream, after expansion and liberation of the gaseous fraction, is additionally stripped with a gaseous nitrogen stream. The stripping gas stream, after absorbing the additionally liberated components, is likewise subjected to the treatment with the second scrubbing liquid stream, to rewash any concomitantly stripped-out hydrogen sulfide. The two methanol-scrubbing liquid streams are finally combined and fed to a warm regeneration stage effecting the removal and production of the second product gas comprising hydrogen sulfide as well as residual carbon dioxide.
Although a gaseous product fraction enriched in hydrogen sulfide can be obtained with the aid of this conventional process, the degree of recovery of carbon dioxide is extremely poor. It would on the one hand be possible for the gaseous carbon dioxide fraction liberated during the expansion of the second scrubbing liquid stream (methanol preliminarily loaded with carbon dioxide) to be obtained separately, in contrast to the process as depicted in FIG. 1 of German Pat. No. 1,814,064, where it is recovered in a mixture with stripping gas. On the other hand, however, the carbon dioxide fraction liberated during the expansion of the second scrubbing liquid stream (methanol loaded with carbon dioxide and hydrogen sulfide) will in any event be contaminated with stripping gas in the conventional process. Besides, there are still relatively high carbon dioxide proportions in the second product gas fraction comprised of hydrogen sulfide and residual carbon dioxide. Thus, the process of German Pat. No. 1,814,064 would be benefitted by an increased concentration of the hydrogen sulfide in the second product gas as well as in a maximally complete recovery of the carbon dioxide.
The second process, as shown in DAS No. 2,548,700, though more comprehensive by the incorporation of additional process steps, is not a complete answer to the desired objectives. These process steps provide a system wherein the carbon dioxide fraction liberated during the expansion of the second scrubbing liquid stream, the latter being merely preliminarily loaded with carbon dioxide, is not mixed with stripping gas, and that the treatment of the gaseous fraction, liberated during the expansion of the first scrubbing liquid stream containing carbon dioxide and hydrogen sulfide, on the one hand, and the subsequent stripping of the two scrubbing liquid streams, on the other hand, are conducted in two separate columns. Consequently, only the carbon dioxide fraction liberated during stripping is contaminated with stripping gas, while the carbon dioxide converted into the gaseous phase during the expansion of the first scrubbing liquid stream is obtained in an almost pure form. In spite of this measure, the proportion of carbon dioxide obtained in the almost pure form is only about 72%, whereas about 24% is blown off together with the stripping gas and about 4% remains in the concentrated hydrogen sulfide product fraction.